1. Field of the Invention
The invention relates to an anti-creep device for use in a trailer suspension, and more specifically in a low-ride trailer suspension. The invention serves to retard the forward and downward movement of the trailer during loading, loss of air spring pressure, or other "stationary" times to limit the creep of the trailer from a loading dock or parked position.
2. Description of the Prior Art
Many trailers have trailing arm suspensions with air springs to control the relative position of the frame with respect to an axle and also to cushion the relative movement of the axle toward the frame due to bumps in the road. In the loading or unloading of a low-ride trailer, the trailer is typically backed up against a dock by the tractor. The operator then pressurizes the air springs of the low-ride suspension so that the bed of the trailer is level with the loading dock. The operator then lowers the front dolly legs on the trailer until they touch the ground and then removes the tractor. In storage situations the tractor may or may not be connected to the trailer. Although the air springs are pressurized by the tractor's air compressor during transport and stationary times, the air compressor is normally off during loading or unloading operations.
As an empty trailer is loaded, the force from the weight of the goods being transferred to the trailer and the loading equipment, such as a forklift or a and car, lowers the rear portion of the frame rail with respect to the axle. Because the compressor is off during the loading operation, the air pressure in the air spring is not adjusted to compensate for the increased load. While the rear portion of the trailer frame moves downwardly, the front portion of the trailer frame is substantially fixed at the height of the dolly and the trailer frame effectively rotates about the contact point of the dolly with the ground. The downward movement of the rear portion of the trailer frame results in the pivotable movement of the pivotal connection between the trailer frame and the trailing arm. This pivotable movement results in the slight rotation of the trailing arm wheel to move the trailer forward. In other words, the trailer tends to move away from the loading dock. This movement is referred to as "creep." Trailer creep can create hazards for loading. This problem is augmented in low-ride trailers, since the air spring is pressurized to a greater height to compensate for the difference between the dock height and trailer bed height. The background and possible solutions to this problem can be found in PCT/US94/10789, which is herein incorporated by reference.
U.S. Pat. No. 5,333,645, issued Aug. 2, 1994, discloses an apparatus for overcoming trailer creep by providing a dump valve to exhaust air from the air spring when the trailer door is opened. The trailer thus bottoms out on the suspension before loading begins and cannot creep away from the dock. Whereas this system has worked well to prevent creep, not all vehicles are equipped with this system, or cannot use such a system. Also, the door switch sometimes malfunctions and the dump valve does not always exhaust air from the air spring.
Therefore, there is a significant need to reduce or eliminate the creep associated with a trailer during loading. The anti-creep solution must also be simple, reliable and inexpensive if it is to be commercially viable. Further, the anti-creep solution must also not interfere with the normal function of the trailing arm suspension during normal operation thereof.